In semiconductor epitaxial processes, it is required to deposit a layer of doped material of uniform thickness and concentration on a semiconductor wafer. At least three major reactor systems have been used to implement the epitaxial process: (a) barrel, (b) horizontal, and (c) "pancake" reactor systems.
In the barrel reactor, a barrel-like holder is rotatably mounted about its vertical axis within a deposition chamber. The semiconductor wafers are placed (on a small angle from vertical) on the outer surface of the barrel which is rotated as reactant gas flows thereacross in a vertical direction. The wafers may be RF heated or radiantly heated by a battery of quartz lamps, located outside the chamber, to promote epitaxial growth thereon. One such barrel reactor is disclosed in U.S. Pat. No. 3,796,182.
In the horizontal reactor, a susceptor, having the wafers thereon, is placed in an elongated horizontal chamber and reactant gases passed therealong. The susceptor is tilted at an angle of about 2.degree. to accelerate the bulk flow to make the streamwise deposition more uniform. The susceptor is heated inductively which, in turn, heats the wafers to promote epitaxial growth thereon. A typical horizontal reactor is described in U.S. Pat. No. 4,047,496. A two level horizontal reactor is described in an article titled "Chemical-Vapor-Deposition Reactor" S. S. Chern, NASA Technical Support Package, dated August 1979.
The "pancake" type reactor is comprised of a substantially circular, planar susceptor rotatably mounted on a hollow, axially located, shaft. The susceptor, with wafers positioned thereon, is rotated within a chamber and reactant gases are radially directed across the wafers as the susceptor is heated using an RF source placed within the chamber immediately below the susceptor. An example of such a reactor is shown in U.S. Pat. No. 3,757,733.
Although the above-described reactors have proven effective for forming epitaxial thicknesses and dopant concentrations on three inch diameter wafers or less, problems arise when processing larger diameter wafers. Semiconductor wafers on the order of four inches in diameter and larger are being fabricated along with new semiconductor devices which place more stringent requirements on the epitaxial layer deposition and dopant concentrations resulting in a substantial decrease in the tolerance acceptability in epitaxial deposition. For example, maintaining the desired wafer flatness and uniform temperature across the wafer diameter are substantially more difficult to accomplish on the larger diameter wafers.
In addition to the more stringent standards of epitaxial layer thickness uniformity and dopant concentration, it is anticipated that rapid increases in semiconductor device production will occur over the next few years. Accordingly, due to the aforementioned stringent manufacturing standards, wafer size and anticipated increase in semiconductor device production there is a need for new chemical vapor deposition technique which will provide a much higher throughput and improved quality control.